This invention relates to elastomeric polyurethane and/or polyurethane/urea products and more particularly to an effective process for lowering residual free aromatic polyisocyanates in prepolymers by incorporation of 2,4- toluene diisocyanate dimer into the prepolymer.
Aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and poly urethane/urea elastomers. These aromatic diisocyanates generally include compositions such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-methylene bis (phenylisocyanate) and the like. In the preparation of polyurethane and polyurethane/urea elastomers, the aromatic diisocyanates are reacted with a long chain (high molecular weight) polyol to produce a prepolymer containing free isocyanate groups which then may be chain extended with a short chain (low molecular weight) polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer. Long chain, high molecular weight polyols, e.g. those having a molecular weight of above 250, are generally utilized to form the prepolymer and the chain extender is generally a short chain polyol, e.g., C.sub.2 -C.sub.10 polyol, or an aromatic diamine. The long chain, high molecular weight polyol provides flexibility and elastomeric properties to the resin, while the short chain polyol or aromatic diamine provides chain extension or cross-links and adds toughness and rigidity to the resulting elastomeric polymer.
A major problem with mononuclear aromatic diisocyanates, e.g., toluene diisocyanate is that they are toxic and because of their low molecular weight tend to be quite volatile. Because of their toxicity and volatility, extreme care must be taken in the workplace to avoid inhalation and damage to the respiratory tract and contact with the skin.
It is known that residual toluene diisocyanate (free toluene diisocyanate) in prepolymer can be reduced by reducing the isocyanate/hydroxyl ratio of the prepolymer formulation. This method, however, has a detrimental consequence on processing when the prepolymer is chain extended (or cured), namely, the hardness build up rate decreases very significantly which leads to too long a demold time.
Economically, short demold times are very important. Once the reactants are mixed and poured into a mold, it is desirable to remove the product from the mold as soon as possible. The product must have sufficient strength to be handled and normally have sufficient strength to be employed in its intended purpose; however, the physical properties, if desired, usually can be improved by postcuring at elevated temperatures.
Surprisingly, it has been found that by incorporating 2,4 TDI dimer in the prepolymer, low NCO/OH ratio can be used to obtain the desired low residual free toluene diisocyanate content while maintaining the desired hardness build up rate. This additive is the dimer of toluene diisocyanate which is commercially known as Desmodur.TM. TT available from Mobay Chemical Company.
There is some literature on the use of 2,4-toluene diisocyanate dimer in polyurethanes. Semenova, E. P. et al. USSR, Kauch Rezina 1985, (7) 24-6, disclose that an amorphous thermoplastic elastomer from adipic acid-1,4-butanediol - ethylene glycol - 2,4-toluene diisocyanate copolymer was post-treated with 2,4-toluene diisocyanate dimer. This led to the formation of allophanate crosslinks and to increased elasticity modulus and energy capacity. The dimer acted not only as a vulcanizing agent, but also as a filler. 2,4-toluene diisocyanate dimer was used in combination with peroximon F40 (a peroxide) (forming strong bonds) and triallyl cyanurate.
Unlike the instant invention, Semenova et al used the TDI-Dimer as a vulcanizing agent and filler for a finished thermoplastic elastomer. It has been unexpectedly and surprisingly found that when TDI-Dimer is utilized in a prepolymer the free TDI-level may be significantly reduced demoldability, in that prepolymer, while maintaining good cure rates and characteristics.